Water Based Mn-Zn Magnetic Fluid Heat Dissipation Capacity Testing Platform

Manganese zinc magnetic fluid is a temperature sensitive magnetic fluid that can regulate its flow behavior using temperature and magnetic fields. However, there is currently no testing platform for evaluating the heat dissipation ability of this magnetic fluid working fluid by coupling temperature and magnetic fields. This article establishes two experimental testing platforms for applying magnetic fields, namely a circulating pipeline and a temperature equalization plate. Compared with deionized water, evaluate the average temperature and heat dissipation ability of water-based manganese zinc magnetic fluid. The test results show that the heat dissipation start time of the manganese zinc magnetic fluid loop pipe is better than that of deionized water. Under the action of magnetic field (500Gs), the average temperature of the circulating pipeline decreases by 7.2% (heat source power 15W); Under the action of a magnetic field (3000Gs), the thermal resistance of the homogenizing plate (filled with 48% water-based manganese zinc magnetic fluid) decreases by about 16.7% (heat source power 140W). The water-based manganese zinc magnetic fluid working fluid exhibits better heat transfer performance than the deionized water working fluid under high heat source power. The experimental results prove that the designed water-based manganese zinc magnetic fluid working fluid heat dissipation capacity testing platform has reliable experimental quantification results.


Introduction
With the development trend of integrated and high-performance electronic devices, the heating power of electronic devices is increasing exponentially.The use of traditional cooling fluid thermal management technology is affected by complex temperature control environments and cannot effectively solve the problem of high power heat flux density [1][2][3].Nanofluids improve heat transfer efficiency due to the movement and collision between nanoparticles in their base fluid [4][5][6].Magnetic nanofluids, due to their ability to control the movement of magnetic nanoparticles using a magnetic field, have become a new type of functional cooling fluid with temperature field and magnetic field coupling regulation [7][8][9].The magnetic properties of temperature sensitive Mn-Zn magnetic fluids vary significantly with temperature [10][11][12].The project team's preliminary work confirmed that the thermal conductivity of water-based Mn-Zn magnetic fluids (with a mass fraction of 3% of nanoparticles) is 14% higher than that of water-based fluids, and the thermal conductivity of kerosene based Mn-Zn magnetic fluids is even 68% higher than that of the base fluid.However, there is a lack of effective experimental evaluation methods for the cooling and heat dissipation capabilities of Mn-Zn magnetic fluid as a working fluid.Therefore, this article designs two experimental testing platforms, a circulating loop and a temperature equalization plate, and compares them with deionized water working fluid as a reference to systematically evaluate the heat transfer ability of water-based Mn-Zn magnetic fluid for different power simulated heat sources.

Test Bench and Testing Analysis of Heat Dissipation Characteristics of Circulating Pipeline Working Fluid
The design of a circulating pipeline experimental platform for magnetic fluid heat transfer characteristics is shown in figure 1.The circulating pipeline mainly consists of a heating section, a cooling section, and a data acquisition system.The micro pump is started to control the constant flow rate of the working fluid cycle.The permanent magnet applies an external magnetic field as shown in figure 2, and the magnetic field strength at the inlet and outlet of the heating section can be adjusted by adjusting the center distance between a pair of permanent magnet iron blocks and the heating coil.Under the same operating conditions, compare the heat dissipation performance of water-based Mn Zn magnetic fluid and deionized water, as well as the influence of external magnetic field on the heat transfer performance of magnetic fluid.The circulation pipeline mainly consists of a quartz tube with an inner diameter of 6mm and a silicone hose, and the pipeline is wrapped with insulation cotton outside.Thermocouple patches are fixed on the outer wall of the circulation pipeline and arranged at four points: the outlet and inlet of the heating section (according to the flow direction), as well as the outlet and inlet of the cold end.The start-up comparison test of heat dissipation ability between water-based Mn Zn magnetic fluid and deionized water is shown in figure 3. Heating power of 10W, heat source temperature set at 120 ℃; When the set temperature is reached, start the micro pump and chiller.The heating rate of water-based Mn Zn magnetic fluid is higher than that of deionized water.After 15 minutes, the temperatures of both working fluids tend to stabilize, with a stable temperature of about 57 ℃ at the magnetic fluid heat source and 63 ℃ in deionized water working fluid.It can be seen that water-based Mn-Zn magnetic fluids exhibit superior heat dissipation ability and start-up rate compared to deionized water due to their higher thermal conductivity.The difference in heat dissipation ability between the two is particularly significant as the heating power increases.At 30W, the outlet temperature of the water-based Mn-Zn magnetic fluid at the hot end decreases by 10 ℃ compared to deionized water.
The magnetic field adopts two 500Gs bar magnets placed opposite each other, as shown in figure 2. A comparative experiment was conducted on the heat dissipation ability of the two working fluids in a constant circuit with a flow rate of 0.2L/min, different heating powers, with and without magnetic field effects.After the circuit temperature is balanced and stable, the average temperature of the cold end and hot end inlet and outlet is used to compare the heat transfer characteristics of the two working fluids.The experimental test results are shown in figure 4. At various heating powers, the average temperature of the circuit with applied magnetic field is generally lower than that of the operating condition without applied magnetic field.Therefore, the application of a magnetic field enhances the heat transfer ability of Mn-Zn magnetic fluid.The test variables of the magnetic field circulation pipeline working fluid heat dissipation characteristics test bench include: simulated heat source power, magnetic field strength, working fluid type, and loop working fluid circulation speed.

Test Bench and Test Analysis of Temperature Equalization Characteristics of Working Medium under the Action of Magnetic Field
Schematic diagram of a magnetic fluid homogenization plate experimental testing platform, which includes a homogenization plate testing unit, a constant temperature water cooling system, a liquid filled vacuum system, a data monitoring and acquisition system, and a simulated heat source heating system, as shown in figure 5.The overall testing unit of the homogenization plate is tightened with fastening bolts to reduce the contact thermal resistance between the homogenization plate and the cold plate, as well as the simulated heat source.The working fluid is directly vacuum filled into an aluminum homogenizing plate using deionized water or Mn-Zn magnetic fluid at a 48% filling rate.Place 3 thermocouples above the simulated heat source, and 5 K-type thermocouples at the contact interface between the cold plate and the uniform temperature plate.Circular magnet(The surface magnetic field strength is 3000Gs) and the electrical wooden block is processed to place the circular magnet in a position; Adjust the magnetic field intensity acting on the bottom surface of the homogenizing plate by using the distance between the circular magnet and the bottom surface of the homogenizing plate, as shown in figure 6; Position 1: The circular magnet is tightly attached to the lower surface of the homogenizing plate, Position 2: The distance between the circular magnet and the bottom surface of the homogenizing plate is 5mm, and the magnetic field strength acting on the bottom surface of the homogenizing plate is 550Gs; Position 3: The distance between the circular magnet and the bottom  The axial thermal resistance curve of the magnetic fluid temperature equalization plate in the counter gravity state (with the hot end at the top and the cold end at the bottom) varies with power, as shown in figure 7. The thermal magnetic convection has a significant improvement in the heat transfer performance of the magnetic fluid homogenizing plate under the counter gravity state.After the power at the hot end exceeds 50W, the thermal resistance under the action of the magnetic field is significantly lower than that without the magnetic field.When the power at the hot end is 100W, the existence of thermal magnetic convection reduces the equilibrium temperature of the magnetic fluid homogenizing plate by 13.67 ℃ compared to without a magnetic field, and the temperature reduction reaches 19.8%;When the heating power is 140W, the axial thermal resistance is 0.30 ℃/W with a magnetic field, while the axial thermal resistance without a magnetic field is 0.37 ℃/W, and the axial thermal resistance of deionized water is 0.36 ℃/W.The experimental platform has verified that the effect of magnetic field improves the heat transfer performance of the magnetic fluid temperature equalization plate in the counter gravity state.The average temperature performance of the average temperature plate is represented by the average temperature coefficient TUI as: (1) In the equation (1): Ti is the temperature of the i-th temperature measurement point at the cold end contact interface; n is the number of temperature measurement points, where n=5.
The TUI curve of the average temperature coefficient of the temperature equalization plate under different magnetic pole distances in the gravity state (cold end above, hot end below) represents three magnetic field intensities, as shown in figure 8. Overall, the average temperature performance of the temperature equalization plate shows a gradually decreasing trend with increasing heating power.From figure 8, it can be seen that the average temperature performance is the worst at a distance of 0mm from the bottom of the average temperature plate, with TUI higher than the other two magnetic field strengths in the power range of 20-140W.The influence trend and heat transfer performance of magnetic fluid homogenization plates with magnetic field distances of 5mm and 10mm are similar.When the heat source power is 140W, the TUI of magnetic field distances of 0mm, 5mm, and 10mm are 0.29, 0.21, and 0.18, respectively.The stronger the external magnetic field, the better the temperature uniformity of the water-based Mn-Zn magnetic fluid homogenization plate.

Theoretical Analysis
The finite element simulation results of the magnetic field line distribution formed by two permanent magnets N-N placed opposite each other in the circulating pipeline experimental platform are shown in figure 9.The different colors in the figure represent the difference in magnetic flux density, and it can be seen that the magnetic flux density is the highest near the outer edge of the magnet.At the same time, under the joint action of the two magnets, a larger magnetic flux density is also formed in the magnetic fluid circuit.The black closed curve is a magnetic field line, whose density represents the magnitude of magnetic induction intensity, and its distribution can also indicate the uniformity of the magnetic field.It can be seen that the magnetic field lines in the magnetic fluid pipeline area are basically parallel, indicating the formation of a relatively uniform magnetic field parallel to the flow direction of the circuit in this area.
The finite element simulation results of the convection state of water-based Mn-Zn magnetic fluid inside the homogenization plate with/without magnetic field effect on the homogenization characteristic experimental platform, as shown in figures 10 and 11.When there is no magnetic field, the magnetic fluid is in a natural convection state under the action of gravity; When there is an annular magnet, the magnetic fluid is in a thermal magnetic convection state.Compared to natural convection, thermal magnetic convection makes the flow behavior of the working fluid inside the homogenizing plate more intense.The presence of a magnetic field accelerates the reflux process of the working fluid on the condensation surface [13][14][15].The cycle of evaporation condensation evaporation of the magnetic fluid working fluid is spread throughout the entire homogenizing plate, and thermal magnetic convection enhances the heat transfer ability of the homogenizing plate and improves the uniformity of the temperature distribution of the homogenizing plate.

Conclusion
This article compares and evaluates the heat transfer performance and temperature equalization characteristics of water-based Mn-Zn magnetic fluid by building two experimental testing platforms, namely the circulating pipeline and the temperature equalization plate.The research results are as follows: (1) The test results of the circulating pipeline test bench indicate that under the action of a magnetic field (two 500Gs permanent magnets), water-based Mn-Zn magnetic fluid exhibits better heat dissipation ability and start-up performance than deionized water working fluid.Under the action of a magnetic field, when the heat source temperature is set at 120 ℃, the starting time of the magnetic fluid working fluid is about 30% faster than that of the deionized water working fluid; Under steady-state conditions, when the simulated heat source power is 15W and the circulating flow rate of the working fluid is 0.2L/min, the average temperature of the circulating circuit of water-based Mn-Zn magnetic fluid working fluid is 7.2% lower than that of deionized water working fluid.
(2) Under the action of a magnetic field (annular permanent magnet 3000Gs), the temperature equalization characteristics of the homogenizing plate test bench was tested.The test results showed that the heat transfer ability of water-based Mn-Zn magnetic fluid working fluid was better than that of deionized water working fluid (with a filling rate of 48%).When the heat source power was 140W, the thermal resistance decreased by 16.7%, and the magnetic fluid working fluid showed anti gravity effect; Try to verify that the magnetic field strength affects the average temperature coefficient (TUI) of the filled water-based Mn-Zn magnetic fluid homogenizing plate, rather than the stronger the magnetic field, the better.
(3) The circulating pipeline test bench and the homogenization plate homogenization characteristic test bench can comprehensively evaluate the heat transfer and homogenization characteristics of magnetic fluid working fluids.At the same time, they also provide a testing method for the coupling of

Figure 3 .
Figure 3. Comparative experiment on heat dissipation and start-up characteristics of deionized water and water-based Mn Zn magnetic fluid.

Figure 4 .
Figure 4. Average temperature of water-based Mn-Zn magnetic fluid in pipelines with and without magnetic field effect.

Figure 5 .
Figure 5. Magnetic fluid temperature equalization plate test system.

Figure 7 .
Figure 7. Counter gravity state, axial contact thermal resistance between the uniform temperature plate and the heat source.

Figure 8 .
Figure 8. Gravity state, variation of average temperature coefficient of temperature plates with different distances of magnetic pole.

Figure 9 .
Figure 9.The magnetic distribution along the pipeline under the effect of a pair N-N magnets.

Figure 10 .
Figure 10.Natural convection of magnetic fluid in a uniform temperature plate.

Figure 11 .
Figure 11.Thermomagnetic convection of magnetic fluid in a uniform temperature plate.